ABSTRACT Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is the most common inherited cause of small vessel disease (SVD), a condition that results in stroke and dementia. Our mission is to understand the molecular pathology of CADASIL in order to define pathways which can be targeted to treat SVD. A majority of CADASIL is caused by cysteine-altering mutations in NOTCH3, suggesting that local redox alteration of the protein is pathogenic. Consequently, altered forms of NOTCH3 may lead to the primary pathological features of CADASIL, which include massive protein accumulation, cell separation, and smooth muscle cell death. In preliminary studies, we identify a novel N-terminal fragment (NTF) of NOTCH3. NTF is a 41 amino acid fragment derived from proteolysis of full-length NOTCH3 that is abundantly expressed in arteries of CADASIL patients. Moreover, NTF is confined to the media of arteries, a region that suffers intense cellular damage in SVD. NTF injected into brains of mice associates with vessels and causes profound small vessels narrowing. Finally, transgenic mice making NTF develop adult onset neurological signs and early death, suggesting the NTF initiates SVD. In this proposal, we hypothesize that NTF is generated by proteolysis of misfolded NOTCH3 and collaborates with full length NOTCH3 to exert neuropathology. Two aims will be pursued: (1) we will characterize the molecular features of NOTCH3 that lead to the generation of NTF, and (2) we will test whether NOTCH3 full length protein is required for NTF to promote SVD of the brain. These studies of CADASIL will potential implicate NTF as a novel pathological protein factor in the genesis of SVD.